The present invention relates to cameras.
In particular, the present invention relates to single lens reflex cameras and especially to light-measuring systems thereof.
At the present time it is well known in cameras of this type to direct to the viewfinder and to measure light which has already traveled through the objective of the camera. The present invention relates in particular to this type of camera.
In general, full utility is achieved from a single lens reflex camera by utilizing interchangeable optical systems therewith so that such a camera has a wide range of application. In order to increase the effective use of such cameras it has recently become common to provide such cameras with interchangeable viewfinders as well as with interchangeable objectives, so as to be capable of expanding the range of use of such a camera. However, in connection with interchangeable viewfinders difficulties have been encountered where the light is measured after traveling through the objective because the photosensitive element which receives the light to participate in the measurement thereof is conventionally situated adjacent the pentaprism of the viewfinder. Thus, with cameras of this type along with interchangeable viewfinders it has also been necessary to provide modified light-measuring devices.
Requirements of this latter type have come about because a particular photosensitive element for each of the number of interchangeable viewfinders not only is uneconomical but also deteriorates the reliability of the electrical connection of the photosensitive element. Furthermore, it is required that the light-measuring or photometric characteristics of the interchangeable viewfinders coincide perfectly with each other.
In order to fulfill requirements of this latter type, improvements in the arrangement of the photosensitive element have been attempted such as situating the photosensitive element in the mirror box instead of adjacent the pentaprism. However, changing the location of the light-measuring device in this way is disadvantageous as compared to situating the photosensitive element of the light-measuring device adjacent the pentaprism in that the distribution of light to a photosensitive element situated in a mirror box is extremely uneven depending upon such factors as whether the light-measuring optical system includes particular lenses such as Fresnel lenses or not and particularly as to whether there is a lack of coincidence in the operative association between the diaphragm of one and the same interchangeable lens which is to be used with different types of camera bodies. Disadvantages of this latter type result in the fact that the photosensitive element situated in the mirror box is relatively small and an extremely spot-type of photometering often takes place. With a photographic camera of the automatic exposure type wherein spot photometering is utilized, accurate automatic exposure regulation is not possible unless a memory lock device is additionally provided when the image of the object to be photographed misses (is not received by) the image frame and this memory lock device is operated separately during the photographing operations. In addition, there is a problem in connection with the light-measuring optical system in that there is a possible undesirable influence of light entering through the ocular of the viewfinder. This latter influence should of course be reduced to a minimum. Where the photosensitive element is situated adjacent to the pentaprism, this latter type of undesirable influence is very likely to be encountered, so that this latter factor represents a considerable disadvantage in connection with a type of system where the photosensitive element is adjacent the pentaprism.